Elimination of methyl mercaptan from the kraft pulping process with polythionate and polysulphide solutions

The absorption of methyl mercaptan from the gas phase into an aqueous polythionate solution prepared by mixing sulphite and sulphide solutions under different conditions (Wackenroder's reaction) was studied. For comparison, the absorption of methyl mercaptan gas by polysulphide solutions was also studied but to a minor extent. The experiments showed that it is possible to prepare a polythionate solution from sulphite and sulphide solutions, which are already available in the kraft pulp mill, and that this solution has a considerably better methyl mercaptan elimination capacity than the usually used NaOH solution. The formation of polythionates was assessed by the ability of the sulphite-sulphide mixture to absorb methyl mercpatan better than a pure NaOH solution at the same alkalinity, as no suitable analytical method for determining the amounts of the various polythionate ions was available. For the absorption process, a very simple reaction vessel was used in order to increase the differences between the absorption abilities of the different solutions studied. The absorption ability of a sulphite-sulphide mixture increased with decreasing pH at the time of preparation and it seems that the dissolved H2S is an active species in the reaction with sulphite for polythionate formation. A lower temperature and longer reaction time during preparation increased the absorption of methyl mercaptan. High concentrations of chemicals during the preparation as well as during the absorption increased the absorption ability of the sulphite-sulphide mixture. The ability of polythionate solution to absorb methyl mercaptan increased with increasing pH at the time of absorption, i.e. when the concentration of hydroxide ions increased. Two different methyl mercaptan gas inlet methods were compared, and it was found that bubbling the methyl mercaptan through the absorbing solution was as expected substantially more effective than merely allowing the gas to flow past the surface of the absorbing solution. Studies on the absorption of methyl mercaptan gas with polysulphide solution showed that higher concentrations of polysulphide sulphur substantially improved the absorption of methyl mercaptan when the gas was bubbled through the absorbing solution. The concentrated polysulphide solutions also had a substantially better ability to absorb mercaptan than the best polythionate solutions produced.